Recent developments in the aviation industry have contributed to a considerable increase in the amount of electrical equipment on board aircraft. Further, the appearance of large-capacity aircraft and the desire to limit the impact of flights on the environment have urged aircraft manufacturers to look for ways of minimizing the weight of said aircraft.
Concerning the electric cables used in aircraft, such tendencies have given rise to the production of cables that are capable of transmitting ever higher voltages without, if possible, modifying their weights or dimensions. Under such conditions, the consequence of increasing the voltage is to generate a phenomenon of partial electric discharges in the cables by avalanche ionization of the air. In this phenomenon, when electrons are subjected to an intense electrical field, they acquire sufficient energy to cause the ionization of neutral molecules (for example the molecules of the gases constituting the air) and thus create new free electrons, which are also capable of ionizing other neutral molecules. When the voltage is sufficient, an electric arc is produced.
Said phenomenon, also known as the corona effect, is influenced by various factors such as the nature and the temperature of the material in which the discharge occurs and the pressure of the ambient air. When the pressure of the air drops, the voltage at which discharge appears also drops. An airplane generally flies at an average altitude of 10 000 meters, where the pressure is approximately 200 hPa [hectoPascal] to 300 hPa. Thus, flight conditions favor the appearance of the corona effect.
When a partial discharge occurs in a cable comprising a conductive core covered with an insulating material, that material is subjected to various stresses:                a thermal stress due to a local increase in the temperature in the zone where the partial discharge occurs;        chemical stresses due to the generation of ozone and nitric acid during the partial discharge; and        mechanical stresses due to erosion of the surface of the material and enlargement of pores within it.        
Said stresses all cause deterioration of the material, from simple premature aging to the appearance of cracks.
Patent application US 2004/0031620 describes an electric cable in which the insulating material surrounding the conductive core is a matrix based on polyamideimide or polyesterimide to which a metal oxide, titanium dioxide, has been added. That material can prevent the corona effect.
However, certain applications require the use of a material having both electrical insulation properties and good temperature resistance, such as PTFE.
Unfortunately, it has not yet been possible to introduce metal oxides (also known as fillers) such as titanium dioxide into extruded PTFE in quantities that allow an anti-corona effect to be obtained. An introduction of that type gives rise to major difficulties:                the presence of fillers in the PTFE has the result of rendering the PTFE porous, resulting in a low density PTFE material being obtained. However, in order not to encourage the corona effect, it is necessary to limit the quantity of air present in the material and, as a consequence, to minimize the number of pores present therein; and        the presence of a filler in PTFE also gives rise to problems during extrusion of the material, such as an increase in the extrusion pressure or the risk of breakage during calendering. Such phenomena may be avoided by adding lubricant to the composition. However, during the drying step, the lubricant also has a tendency to create pores in the material in which it is incorporated, which then results in a low-density material.        